The present invention relates to a method for producing pure monocarbides, or pure mononitrides or carbonitrides of one or a plurality of the metals titanium, vanadium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, thorium, uranium, plutonium, or americium.
Carbides, nitrides and carbonitrides are gaining more and more importance in the high-temperature art. The thermodynamically most stable and technically most interesting carbides are formed practically exclusively from the transition metals of the IVth, Vth and VIth groups of the Periodic Table, as well as from the actinide elements. In addition to tungsten carbide (WC) and titanium carbide (TiC), such compounds as niobium carbide (NbC), tantalum carbide (TaC), vanadium carbide (VC) and molybdenum carbide (MoC) are increasingly used as solid phases in hard metals. Also, carbide cermets are used in the manufacture of gas turbines. Further, significant improvements in mechanical properties are obtained for composite metallic materials with fibers, e.g. whisks, of SiC, B.sub.4 C and ZrC.
The prior art has also used nitrides as hard substances, e.g. for protective tubes around thermoelements, crucibles, etc. Previously, carbides were produced by carburizing the powdered metals or by reducing the oxides with carbon. In the past, nitrides were manufactured by nitrating metal oxides in the presence of carbon, by nitrating metals, by decomposing ammonia compounds or by deposition from the gaseous phase.
High demands are placed on nuclear fuels for high output nuclear reactors such as fast breeder reactors. For example, the nuclear fuel elements must be extremely pure and compatible with fuel element cladding materials. Typical of such cladding materials are stainless steels.
At the high operating temperatures (700.degree.C and more) and the desired long fuel element service life which result in high fuel consumption (80,000 to 100,000 megawatt.days per ton or more) prior experience has shown that carbides and nitrides of uranium and plutonium are preferable to the corresponding oxides as nuclear fuels.
It has been found that of the carbides and nitrides, the monocarbides and the mononitrides show the best compatibility with the cladding materials used in nuclear reactors.
In the past, the manufacture of pure uranium monocarbide (UC), or pure uranium mononitride (UN) from uranium oxide has been effected by a number of different methods. For example, in one method, uranium oxide is reduced to metallic uranium with the aid of calcium. The metallic uranium is then reacted in comminuted form, e.g. in the form of chips or powder, at approximately 1000.degree.C with methane according to the formula: ##EQU1##
In another prior art method, after the metallic uranium is formed, it is reacted with nitrogen at less than 1000.degree.C and the reaction product is subsequently disproportionated in a vacuum at temperatures over 1000.degree.C to UN and nitrogen according to the formula: ##EQU2##
These manufacturing processes, however, are not economically feasible and are unsuited for commercial utilization where production of larger quantities of fuel is required.
It is also known to precipitate ammonium diuranate (ADU) and/or ammonium uranyl carbonate (AUC) from aqueous solutions containing uranium (VI) ions. For example, ammonium diuranate can be precipitated from uranyl nitrate solutions containing nitric acid by adding ammonia. Similarly, ammonium uranyl carbonate (AUC) can be precipitated from uranyl nitrate solutions containing nitric acid by adding ammonium carbonate. The resulting deposits are then separated from the solution, dried and calcined (See, for example, British Pat. No. 1,096,592). The calcined deposit can then be reduced to uranium dioxide. Uranium/plutonium dioxide can be produced in a similar manner. Uranium dioxide produced according to the ADU/AUC method, however, produces only impure reaction products when it is mixed with carbon and reacted by carbothermal reduction to form uranium monocarbide or when it is mixed with carbon and reacted in a stream of nitrogen to form uranium mononitride. The uranium dioxide used as the reaction compound is superstoichiometric (UO.sub.2 2.sub.+x) and slow to react, a so-called altered oxide. Analytical difficulties make it impossible to accurately determine oxygen in the oxide. This determination is required to calculate the quantity of carbon to be added, and thus the required uranium oxide/carbon mixture cannot be produced with precision. For this reason, and due to the partial formation of carbon dioxide during the carbothermal reduction, the resulting uranium monocarbides contain many impurities in the form of uranium dicarbides (UC.sub.2) as a second phase in the range of several percent by weight. The UC.sub.2 must be converted to uranium sesquicarbide (U.sub.2 C.sub.3) by further thermal treatment because the uranium sesquicarbide exhibits a better compatibility behavior with respect to cladding materials than does the dicarbide. The oxygen impurities in the resulting carbides amount to approximately 0.4 to 0.5 percent by weight. For similar reasons, the production of uranium mononitride also does not result in a single-phase reaction product. Oxygen impurities in the mononitrides are frequently present in the form of uranium dioxide as the second phase in the range of a few percent by weight. Free carbon is also frequently found in the monocarbide and mononitride end products. Moreover, temperatures in the area of 1700.degree. to 1800.degree.C are required for the carbothermal reduction in vacuo and for the reaction in a stream of nitrogen.
It has also been proposed to treat a solution containing uranium (IV) and plutonium (III) ions with a solution containing oxalate ions to precipitate a mixture of U(IV) and Pu(III) oxalates from the solution (German Published Patent Appln. No. 1,223,353). The precipitates deposit easily and can be filtered easily. The precipitated oxalates are then calcined in air and are reduced with the aid of hydrogen to a mixture of uranium and plutonium dioxide or are calcined directly in nitrogen or hydrogen with the formation of dioxides. The dioxides resulting in this manner are altered dioxides, are slow to react, and require high reaction temperatures to be converted to monocarbides or mononitrides.
It is therefore a primary object of the present invention to provide a process with which the drawbacks of the known methods can be avoided and highly pure monocarbides, mononitrides and carbonitrides of the metals of the group including titanium, vanadium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, thorium, uranium, plutonium, or americium can be produced simply, safely and economically at relatively low temperatures in degrees of purity which can be reproduced